U.S. patent application number 13/203751 was filed with the patent office on 2012-01-12 for pre-collapsed cmut with mechanical collapse retention.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Peter Dirksen.
Application Number | 20120010538 13/203751 |
Document ID | / |
Family ID | 42083896 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120010538 |
Kind Code |
A1 |
Dirksen; Peter |
January 12, 2012 |
PRE-COLLAPSED CMUT WITH MECHANICAL COLLAPSE RETENTION
Abstract
A CMUT transducer cell suitable for use in an ultrasonic CMUT
transducer array has a membrane with a first electrode, a substrate
with a second electrode, and a cavity between the membrane and the
substrate. The CMUT is operated in a precollapsed state by biasing
the membrane to a collapsed condition with the floor of the cavity,
and a lens is cast over the collapsed membrane. When the lens
material has polymerized or is of a sufficient stiffness, the bias
voltage is removed and the lens material retains the membrane in
the collapsed state.
Inventors: |
Dirksen; Peter;
(Valkenswaard, NL) |
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
Eindhoven
NL
|
Family ID: |
42083896 |
Appl. No.: |
13/203751 |
Filed: |
February 10, 2010 |
PCT Filed: |
February 10, 2010 |
PCT NO: |
PCT/IB10/50614 |
371 Date: |
September 26, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61155988 |
Feb 27, 2009 |
|
|
|
Current U.S.
Class: |
601/2 ;
367/140 |
Current CPC
Class: |
A61B 8/00 20130101; B06B
1/0292 20130101; A61B 8/4483 20130101 |
Class at
Publication: |
601/2 ;
367/140 |
International
Class: |
A61N 7/00 20060101
A61N007/00; B06B 1/02 20060101 B06B001/02 |
Claims
1. A CMUT transducer cell comprising: a substrate; a first
electrode attached to the substrate; a movable membrane formed in
spaced relationship to the first electrode; a second electrode
attached to the membrane; and a retention member, overlaying the
movable membrane when the membrane is in a precollapsed state which
acts to retain the membrane in its precollapsed state in the
absence of a bias voltage.
2. The CMUT transducer cell of claim 1, wherein the retention
member is a transducer lens.
3. The CMUT transducer cell of claim 2, wherein the transducer lens
is made of one of polydimethyl siloxane, PDMS, RTV rubber,
urethane, vinyl plastisol, or a thermoplastic elastomer.
4. The CMUT transducer cell of claim 1, wherein the retention
member is cast over the CMUT transducer cell while the membrane is
brought to a precollapsed state by a bias voltage.
5. The CMUT transducer cell of claim 1, wherein the retention
member is cast over the CMUT transducer cell while the membrane is
brought to a precollapsed state by application of pressure to the
membrane.
6. The CMUT transducer cell of claim 5, wherein the pressure is
atmospheric pressure.
7. A CMUT transducer array comprising a plurality of the CMUT
transducer cells of claim 1, wherein the retention member further
comprises an acoustic lens formed over the array of CMUT transducer
cells.
8. The CMUT transducer array of claim 7, wherein the acoustic lens
provides the array with a fixed focus within a field of interest
which is less than that of a plane wave.
9. The CMUT transducer array of claim 7, wherein the transducer
array is formed by a CMOS-compatible semiconductor process; wherein
the transducer array is incorporated in a diagnostic ultrasound
probe; and wherein the ultrasound probe further comprises an
electronic circuit coupled to the transducer array for operating
the array, wherein the electronic circuit is formed by a
CMOS-compatible semiconductor process.
10. The CMUT transducer array of claim 9, wherein the electronic
circuit further comprises a microbeamformer circuit.
11. The CMUT transducer array of claim 10, wherein the transducer
array and the microbeamformer circuit are further fabricated on the
same semiconductor substrate.
12. The CMUT transducer array of claim 7, wherein the transducer
array is formed by a CMOS-compatible semiconductor process; wherein
the transducer array is incorporated in a therapeutic ultrasound
probe; and wherein the ultrasound probe further comprises an
electronic circuit coupled to the transducer array for operating
the array, wherein the electronic circuit is formed by a
CMOS-compatible semiconductor process.
13. A CMUT transducer cell comprising: a substrate; a first
electrode attached to the substrate; a movable membrane formed in
spaced relationship to the first electrode; a second electrode
attached to the membrane; and a retention member, overlaying the
movable membrane when the membrane is in a partially precollapsed
state which acts to retain the membrane in its partially
precollapsed state in the absence of a bias voltage.
14. The CMUT transducer cell of claim 13, further comprising a bias
voltage applied to the first and second electrodes, wherein the
retention member and the bias voltage act to retain the membrane in
a fully precollapsed state.
15. The CMUT transducer cell of claim 14, wherein the retention
member is a transducer lens.
Description
[0001] This invention relates to medical diagnostic ultrasonic
imaging and, in particular, to ultrasound probes which use
capacitive micromachined ultrasonic transducers (CMUTs).
[0002] The ultrasonic transducers used for medical imaging have
numerous characteristics which lead to the production of high
quality diagnostic images. Among these are broad bandwidth and high
sensitivity to low level acoustic signals at ultrasonic
frequencies. Conventionally the piezoelectric materials which
possess these characteristics and thus have been used for
ultrasonic transducers have been made of PZT and PVDF materials,
with PZT being the most preferred. However the ceramic PZT
materials require manufacturing processes including dicing,
matching layer bonding, fillers, electroplating and
interconnections which are distinctly different and complex and
require extensive handling, all of which can result in transducer
stack unit yields which are less than desired. Furthermore, this
manufacturing complexity increases the cost of the final transducer
probe. As ultrasound system mainframes have become smaller and
dominated by field programmable gate arrays (FPGAs) and software
for much of the signal processing functionality, the cost of system
mainframes has dropped with the size of the systems. Ultrasound
systems are now available in inexpensive portable, desktop and
handheld form. As a result, the cost of the transducer probe is an
ever-increasing percentage of the overall cost of the system, an
increase which has been accelerated by the advent of higher
element-count arrays used for 3D imaging. Accordingly it is
desirable to be able to manufacture transducer arrays with improved
yields and at lower cost to facilitate the need for low-cost
ultrasound systems.
[0003] Recent developments have led to the prospect that medical
ultrasound transducers can be manufactured by semiconductor
processes. Desirably these processes should be the same ones used
to produce the circuitry needed by an ultrasound probe such as a
CMOS process. These developments have produced micromachined
ultrasonic transducers or MUTs. MUTs have been fabricated in two
design approaches, one using a semiconductor layer with
piezoelectric properties (PMUTs) and another using a diaphragm and
substrate with electrode plates that exhibit a capacitive effect
(CMUTs). The CMUT transducers are tiny diaphragm-like devices with
electrodes that convert the sound vibration of a received
ultrasound signal into a modulated capacitance. For transmission
the capacitive charge applied to the electrodes is modulated to
vibrate the diaphragm of the device and thereby transmit a sound
wave. Since these devices are manufactured by semiconductor
processes the devices generally have dimensions in the 10-200
micron range, but can range up to device diameters of 300-500
microns. Many such individual CMUTs can be connected together and
operated in unison as a single transducer element. For example,
four to sixteen CMUTs can be coupled together to function in unison
as a single transducer element. A typical 2D transducer array
currently will have 2000-3000 piezoelectric transducer elements.
When fabricated as a CMUT array, over one million CMUT cells will
be used. Surprisingly, early results have indicated that the yields
on semiconductor fab CMUT arrays of this size should be markedly
improved over the yields for PZT arrays of several thousand
transducer elements.
[0004] CMUTs were initially produced to operate in what is now
known as an "uncollapsed" mode. Referring to FIG. 1, a typical
uncollapsed CMUT transducer cell 10 is shown in cross-section. The
CMUT transducer cell 10 is fabricated along with a plurality of
similar adjacent cells on a substrate 12 such as silicon. A
diaphragm or membrane 14 which may be made of silicon nitride is
supported above the substrate by an insulating support 16 which may
be made of silicon oxide or silicon nitride. The cavity 18 between
the membrane and the substrate may be air or gas-filled or wholly
or partially evacuated. A conductive film or layer 20 such as gold
forms an electrode on the diaphragm, and a similar film or layer 22
forms an electrode on the substrate. These two electrodes,
separated by the dielectric cavity 18, form a capacitance. When an
acoustic signal causes the membrane 14 to vibrate the variation in
the capacitance can be detected, thereby transducing the acoustic
wave into a corresponding electrical signal. Conversely, an a.c.
signal applied to the electrodes 20,22 will modulate the
capacitance, causing the membrane to move and thereby transmit an
acoustic signal.
[0005] Due to the micron-size dimensions of a typical CMUT,
numerous CMUT cells are typically fabricated in close proximity to
form a single transducer element. The individual cells can have
round, rectangular, hexagonal, or other peripheral shapes. FIG. 3
is a topographical image produced by an optical interferometer of a
circular CMUT cell of the present invention. FIG. 4 is an
interferometric image of an array of circular CMUT cells. The CMUT
cells can have different dimensions so that a transducer element
will have composite characteristics of the different cell sizes,
giving the transducer a broad band characteristic. Generally such
cell size differentiation is not necessary, as most CMUTs normally
have a bandwidth of 100% or more of the applied signal
bandwidth.
[0006] The CMUT is inherently a quadratic device so that the
acoustic signal is normally the harmonic of the applied signal,
that is, the acoustic signal will be at twice the frequency of the
applied electrical signal frequency. To prevent this quadratic
behavior a bias voltage is applied to the two electrodes which
causes the diaphragm to be attracted to the substrate by the
resulting coulombic force. This is shown schematically in FIG. 2,
where a DC bias voltage V.sub.B is applied to a bi as terminal 24
and is coupled to the membrane electrode 20 by a path which poses a
high impedance Z to a.c. signals such as an inductive impedance.
A.c. signals are capacitively coupled to and from the membrane
electrode from a signal terminal 26. The positive charge on the
membrane 14 causes the membrane to distend as it is attracted to
the negative charge on the substrate 12. The CMUT cell only weakly
exhibits the quadratic behavior when operated continuously in this
biased state.
[0007] It has been found that the CMUT is most sensitive when the
membrane is distended so that the two oppositely charged plates of
the capacitive device are as close together as possible. A close
proximity of the two plates will cause a greater coupling between
acoustic and electrical signal energy by the CMUT. Thus it is
desirable to increase the bias voltage V.sub.B until the dielectric
spacing 32 between the membrane 14 and substrate 12 is as small as
can be maintained under operating signal conditions. In constructed
embodiments this spacing has been on the order of one micron or
less. If the applied bias voltage is too great, however, the
membrane can contact the substrate, short-circuiting the device as
the two plates of the device are stuck together by VanderWals
forces. This sticking can occur when the CMUT cell is overdriven,
and can vary from one device to another with the same bias voltage
V.sub.B due to manufacturing tolerance variations. While permanent
sticking can be reduced be embedding the device electrodes in an
electrical isolation layer (e.g., silicon nitride), the
nonlinearity of operation between collapsed and uncollapsed states
is an inherent disadvantage when trying to operate an uncollapsed
CMUT in a range of maximal sensitivity.
[0008] Even when the membrane is biased to cause a very small
sub-micron dielectric spacing, the sensitivity of the CMUT can be
less than that which is desired. This is due to the fact that,
whereas the charge at the center 32 of the membrane is relatively
close to and will move considerably in relation to the opposing
charge, the charge at the periphery 34 of the membrane where the
membrane is supported by the support 16 will move very little and
hence have little participation in the transduction of signals by
the device. One approach to eliminating this disparity has been to
use a small membrane electrode 20 which does not extend to the
supports 16. This restricts the charge on the membrane electrode to
the center of the device where it will participate strongly in the
motion of the membrane and hence the transduction by the device.
There still must be one or more electrical conductors to apply the
bias voltage V.sub.B to the membrane electrode 20 and to couple the
a.c. signals to and from the electrode. These electrical conductors
are necessarily very thin, with dimensions that impose undesirably
large impedances on the a.c. signals, thereby limiting the
sensitivity of the device.
[0009] It is an object of the present invention to provide a CMUT
transducer cell with good sensitivity but which is immune to the
membrane sticking problem.
[0010] It is a further object of the present invention to provide a
CMUT transducer cell which can be maintained in an efficient range
of operation with a low bias voltage.
[0011] It is a further object of the present invention to provide a
CMUT transducer cell which operates consistently from lot to lot in
the presence of anticipated manufacturing tolerances.
[0012] It is a further object of the present invention to provide a
CMUT transducer array which can be fabricated with semiconductor
processes that are compatible with those of the integrated
circuitry used to operate the array such as a CMOS process.
[0013] In accordance with the principles of the present invention,
an ultrasonic transducer CMUT cell array is provided which operates
in the "precollapsed" mode. In the precollapsed mode the sticking
problem is avoided because the membrane is continually in contact
with the floor of the cavity of the CMUT cell. Hysteresis is
avoided by use of a range of operation which does not switch
between uncollapsed and precollapsed states and continually
operating in the precollapsed mode. The bias voltage conventionally
needed to maintain the membrane in the precollapsed mode is
replaced by a mechanical structure which physically maintains the
collapsed condition of the membrane. This enables the CMUT to
operate in a favorable range of operation with low operating and
bias voltages. In a preferred embodiment the mechanical structure
which maintains the CMUT cell in the collapsed condition is a lens
of the ultrasonic transducer array.
[0014] In the drawings:
[0015] FIG. 1 is a cross-sectional view of a typical CMUT
transducer cell.
[0016] FIG. 2 is a schematic illustration of the electrical
properties of a typical CMUT cell.
[0017] FIG. 3 is a topographical interferometric image of a CMUT
cell of the present invention.
[0018] FIG. 4 is an interferometric image of an array of circular
CMUT cells.
[0019] FIG. 5 is a cross-sectional view of a CMUT cell constructed
in accordance with the principles of the present invention.
[0020] FIG. 6 illustrates the CMUT cell of FIG. 5 when biased into
a collapsed state.
[0021] FIG. 7 illustrates the CMUT cell of FIG. 6 when the membrane
is retained in the collapsed state by a lens fabricated on top of
the cell.
[0022] FIG. 8 illustrates an array of CMUT cells held in the
precollapsed state by a lens providing a focal characteristic for
the array.
[0023] FIG. 9 illustrates the variation of the coupling
coefficients of precollapsed and uncollapsed CMUT cells with
voltage.
[0024] FIG. 10 illustrates the variation of the measured coupling
coefficient of a constructed embodiment of the present invention
with voltage.
[0025] With reference to FIG. 5, a schematic cross-section of a
CMUT element 5 is depicted. CMUT element 5 includes a substrate
layer 12, an electrode 22, a membrane layer 14, and a membrane
electrode ring 28, the circular form of which is seen in FIGS. 3
and 4. In this example, the electrode 22 is circularly configured
and embedded in the substrate layer 12. In addition, the membrane
layer 14 is fixed relative to the top face of the substrate layer
12 and configured/dimensioned so as to define a spherical or
cylindrical cavity 18 between the membrane layer 14 and the
substrate layer 12. As previously mentioned, the cell and its
cavity 18 may define alternative geometries. For example, cavity 18
could define a rectangular and/or square cross-section, a hexagonal
cross-section, an elliptical cross-section, or an irregular
cross-section.
[0026] The bottom electrode 22 is typically insulated on its
cavity-facing surface with an additional layer (not pictured). A
preferred insulating layer is an oxide-nitride-oxide (ONO)
dielectric layer formed above the substrate electrode and below the
membrane electrode. The ONO-dielectric layer advantageously reduced
charge accumulation on the electrodes which leads to device
instability and drift and reduction in acoustic output pressure.
The fabrication of ONO-dielectric layers on a CMUT is discussed in
detail in European patent application no. 08305553.3 by Klootwijk
et al., filed Sep. 16, 2008 and entitled "Capacitive micromachined
ultrasound transducer." Use of the ONO-dielectric layer is
desirable with precollapsed CMUT, which are more susceptible to
charge retention than are uncollapsed device. The disclosed
components may be fabricated from CMOS compatible materials, e.g.,
Al, Ti, nitrides (e.g., silicon nitride), oxides (various grades),
tetra ethyl oxysilane (TEOS), poly-silicon and the like. In a CMOS
fab, for example, the oxide and nitride layers may be formed by
chemical vapor deposition and the metallization (electrode) layer
put down by a sputtering process. Suitable CMOS processes are LPCVD
and PECVD, the latter having a relatively low operating temperature
of less than 400.degree. C.
[0027] Exemplary techniques for producing the disclosed cavity 18
involve defining the cavity in an initial portion of the membrane
layer 14 before adding a top face of the membrane layer 14. Other
fabrication details may be found in U.S. Pat. No. 6,328,697
(Fraser). In the exemplary embodiment depicted in FIG. 5, the
diameter of the cylindrical cavity 18 is larger than the diameter
of the circularly configured electrode plate 22. Electrode ring 28
may have the same outer diameter as the circularly configured
electrode plate 22, although such conformance is not required.
Thus, in an exemplary embodiment of the present invention, the
electrode ring 28 is fixed relative to the top face of the membrane
layer 14 so as to align with the electrode plate 22 below.
[0028] FIG. 6 shows the CMUT cell of FIG. 5 when biased to a
precollapsed state, in which the membrane 14 is in contact with the
floor of the cavity 18. This is accomplished by applying a DC bias
voltage to the two electrodes as indicated by voltage V.sub.B
applied to the electrode ring 28 and a reference potential (ground)
applied to the substrate electrode 22. While the electrode ring 28
could also be formed as a continuous disk without the hole in the
center, FIG. 6 illustrates why this is not necessary. When the
membrane 14 is biased to its precollapsed state as shown in this
drawing, the center of the membrane is in contact with the floor of
the cavity 18. As such, the center of the membrane 14 does not move
during operation of the CMUT. Rather, it is the peripheral area of
the membrane 14 which moves, that which is above the remaining open
void of the cavity 18 and below the ring electrode. By forming the
membrane electrode 28 as a ring, the charge of the upper plate of
the capacitance of the device is located above the area of the CMUT
which exhibits the motion and capacitive variation when the CMUT is
operating as a transducer. Thus, the coupling coefficient of the
CMUT transducer is improved.
[0029] The membrane 14 may be brought to its precollapsed state in
contact with the floor of the cavity 18 as indicated at 36 by
applying the necessary bias voltage, which is typically in the
range of 50-100 volts. As the voltage is increased, the capacitance
of the CMUT cell is monitored with a capacitance meter. A sudden
change in the capacitance indicates that the membrane has collapsed
to the floor of the cavity. The membrane can be biased downward
until it just touches the floor of the cavity as indicated at 36,
or can be biased further downward to increased collapse beyond that
of minimal contact.
[0030] Another way to bring the membrane 14 to its precollapsed
state is to apply pressure to the top of the membrane. When the
cavity is formed in a partial or complete vacuum, it has been found
that the application of atmospheric pressure of 1 Bar is sufficient
to precollapse the membrane 14 to contact with the floor of the
cavity 18. It is also possible to use a combination of pressure
differential and bias voltage to controllably precollapse the
membrane 14, which is effective with smaller devices that may have
a high atmospheric collapse pressure (e.g., 10 Bar.)
[0031] In accordance with the principles of the present invention,
while the membrane 14 is biased to its precollapsed state as shown
in FIG. 6, a structure is placed or formed above the membrane which
physically retains the membrane in its precollapsed state. In a
preferred embodiment for an ultrasound transducer, the structure
forms the lens 40 of the transducer. A transducer lens normally
fulfills three requirements. One is that the lens provides a
structure which endures wear resistance due to the frictional
contact produced during use of the transducer probe. In effect, the
lens provides a physical cover which protects the underlying
transducer array from physical wear. Second, a lens is
nonconductive and thereby provides electrical insulation between
the electrical elements of the transducer and the patient. Third, a
lens can provide focal properties for the probe. In the example of
FIG. 7, the lens 40 provides a fourth benefit, which is to
physically retain the membrane 14 in its precollapsed state.
[0032] Various materials may be used for the lens material. The
only requirement for the CMUT is that the material be of sufficient
stiffness to retain the membrane in its collapsed state after the
bias voltage is removed. One suitable material is polydimethyl
siloxane (PDMS or RTV rubber). The RTV material is cast over the
CMUT while the bias voltage V.sub.B holds the membrane in its
desired collapsed state. After the RTV polymerizes and is
sufficiently stiff to physically retain the membrane in its
precollapsed state, the bias voltage can be removed and does not
need to be reapplied until the device is biased for operation.
Preferably the lens material is bonded to the areas around each
membrane of the CMUT array. Other materials which may be suitable
for the lens 40 include urethane rubber, vinyl plastisols, and
thermoplastic elastomers.
[0033] By physically retaining the membrane in its precollapsed
state, no bias is necessary to maintain the precollapsed condition
until the operating bias is applied during use of the device. This
means that the CMUT can be operated at lower voltages, which is
advantageous for small, portable ultrasound systems. Furthermore,
adverse effects due to variability in manufacturing and material
characteristics, such as variation in membrane size, stiffness or
cavity depth from lot to lot can be eliminated. These variabilities
may mean that more or less bias voltage is needed to bring the CMUT
to its precollapsed state. The bias voltage is adjusted accordingly
to the desired degree of collapse, and then the lens material holds
the membrane in this state. Thus, each CMUT array can be set up for
the same performance characteristics or its coupling customized
even in the presence of these tolerance variations. Greater
uniformity of the probes in terms of characteristics such as
operating voltage range, acoustic impedance, capacitance, and
coupling coefficient can be achieved.
[0034] FIG. 8 illustrates an example of an implementation of the
present invention where an array of precollapsed CMUTs 5 are held
in the precollapsed state by a lens 42. This lens material exhibits
a slower speed of sound than does the human body, thereby focusing
the array toward a central focal region. Without a focusing lens
the individual CMUTs would all be focused straight ahead and the
array as a whole is focused at infinity. When such an array is
operated to focus it in a desired focal range, a considerable range
of delay is needed to effect the desired focusing. A focusing lens
42 as shown in FIG. 8 can act to give the array a nominal focus
within a desired focal range such as the focal range FR shown in
front of the CMUT array of FIG. 8. With the lens providing this
initial focus, the range of delay needed to change the focus to
specific points or regions within the focal range is decreased. By
placing the lens focal point within the focal range of interest,
the delay requirements of the beamformer can be reduced by a factor
of two as compared with that required for an unfocused plane wave
array. When the delay requirements of the beamformer which operates
the array are reduced, the beamformer will generally be less
expensive and difficult to design and manufacture.
[0035] In an exemplary constructed array of CMUT transducer cells,
the membrane of each CMUT is 50m in diameter or width, the cavity
is 0.33m deep, and the CMUT is 1-5m thick. The lens may be
500-1000m thick and exhibit a stiffness of 1 megaPascal.
[0036] The coupling coefficient of a CMUT in the precollapsed state
is improved and can be varied with lower voltage than is the case
for the CMUT when operating in an uncollapsed state (FIGS. 1 and
2).
[0037] The coupling coefficient of a CMUT cell is a measure of the
efficiency of energy storage by the device and is calculated
as:
k 2 = 1 - C s C T ##EQU00001##
where
C s = Q V ##EQU00002##
and
C T = Q V ##EQU00003##
and Q is charge and V is voltage. Hence, a higher coupling
coefficient is a desirable attribute of an ultrasound transducer,
be it a standard piezoelectric transducer or a CMUT array
transducer. In the case of a CMUT cell, the variation of the
coupling coefficient k.sup.2 with voltage rises in the uncollapsed
state as the voltage increases from zero, as shown by curve 52 in
FIG. 9. As the membrane is biased to more closely approach the
floor of the CMUT cell as shown at 32 in FIG. 2, the coupling
coefficient k.sup.2 changes more rapidly. Hence, the uncollapsed
mode CMUT is operated at this higher voltage range 56 as shown in
FIG. 9. In the precollapsed state, however, the variation of
k.sup.2 with voltage is as shown by curve 54. Here, the variation
of k.sup.2 is steepest at the lower voltages, in the range
indicated by the lower voltage range bracket 58.
[0038] When the applied voltage to the CMUT electrodes is increased
over the uncollapsed region of operation into the collapsed region,
then back again, the coupling coefficient variation will exhibit a
hysteresis effect. Essentially, k.sup.2 will increase along curve
52 as the voltage increases and, when the voltage is decreased
after collapse, the coupling coefficient will decrease back along
curve 54. This hysteresis shows why it is desirable to operate
entirely in one mode or the other. When a precollapsed CMUT is
operated entirely in its precollapsed state it will not have the
hysteresis problem, as shown by the curve 60 in FIG. 10. The curve
10 is drawn along the path of actual measurements of the coupling
coefficient changes of a constructed CMUT cell as the voltage was
changed. The measurement values are indicated by the small circles
along the curve 60. This demonstrates the absence of hysteresis
when a CMUT cell or array of the present invention are operated
continuously in the precollapsed state.
[0039] As previously mentioned, coupling coefficients can be
measured for all varieties of ultrasound transducers, and the
greater the coupling coefficient, the better the performance of the
transducer probe. A typical PZT transducer probe will exhibit an
effective coupling coefficient k.sup.2.sub.Eff (which considers
only the resonance mode of interest) of 0.42. A higher performance
material, single crystal piezoelectric, as described in U.S. Pat.
No. 6,465,937 (Chen et al.), will exhibit an effective coupling
coefficient of about 0.65. Precollapsed CMUT cells of the present
invention can be produced with coupling coefficients in the same
range at that of the best single crystal array probes, and
calculations indicate that even higher coupling coefficients may be
possible.
[0040] Other variations will readily occur to those skilled in the
art. For instance, the lens material does not have to retain the
membrane in a fully precollapsed state. The lens could act to hold
the membrane only partially collapsed toward the floor of the CMUT,
and a small bias voltage used to bring the membrane to a fully
collapsed state. In other words, the fully collapsed state can be
effected in part by a retention member such as the lens material,
and in part by a bias voltage. As used herein the term "collapsed"
or "precollapsed" can mean that the membrane is in contact with the
floor of the CMUT cavity, or only partially distended toward the
floor.
[0041] The CMUT transducer arrays of the present invention are
suitable for use in both diagnostic and therapeutic ultrasound
probes. CMUT arrays of several centimeters in diameter may find use
in high intensity focused ultrasound (HIFU) probes. CMUT
transducers of the present invention may be used in both external
(transthoracic) and indwelling (catheter) ultrasound probes. As
previously mentioned, CMUT arrays of the present invention are
particularly desirable for concurrent fabrication with the
microelectronics needed to operate the probe, as for instance a
CMOS process that is used to produce both the CMUT array and its
microbeamformer on the same or on bonded substrates.
* * * * *